Recently, the demand for high-definition and large-screen display devices such as digital televisions is increasing and thus improvements are being actively made in various fields such as cathode ray tubes (CRT), liquid crystal displays (LCD), and plasma display panels (PDP).
The CRT, which has been widely used in televisions, has a disadvantage that there is an increase in length and weight with an increase of screen size, although it has superior resolution and display quality. For this reason, the CRT is not suitable for large screens of 40 inches or more. The PDP can be implemented as a large screen, and a PDP of 100 inches has already been developed, whereby the PDP, together with the LCD, is at the head of the large-screen display field. However, the PDP has a problem in that the PDP filter is separated from glass due to high temperatures in the containers during shipping. Such separation occurs when a pressure-sensitive adhesive has a very low glass transition temperature (Tg) or its adhesive strength with respect to glass is reduced. To prevent the separation, it is necessary to secure stability at high temperature by increasing the Tg of the pressure-sensitive adhesive, and to increase the adhesive strength with respect to glass. In order to add high Tg and high adhesive strength to an acrylic pressure-sensitive adhesive that is mainly used in the PDP filter, a polar monomer including a carboxyl group should be used. However, the carboxyl group, although it increases the Tg of the pressure-sensitive adhesive, lowers close-adherence with respect to an adherend such as a base film, and builds up initial peeling strength, causing paste marks during reworking. Moreover, such a pressure-sensitive adhesive layer has very high adhesive strength with respect to a glass interface, whereby reworkability is significantly degraded due to high build-up of the peeling strength during high-speed peeling.
To solve the foregoing problems, Japanese Patent Publication No. 2005-263963 discloses a pressures-sensitive adhesive composition including an acrylic copolymer containing an isocyanate group. However, the disclosed technique has to use an isocyanate group-containing acrylic copolymer and an acrylic copolymer containing no isocyanate group as two separate components, resulting in a cumbersome process of mixing those two components before use. Furthermore, in this technique, the contained isocyanate group may react with glass, resulting in excessive increase of adhesive strength with respect to the glass interface.
Japanese Patent Publication No. 2006-143915 discloses a technique for forming a primer layer with resin having a reactive functional group on a polyester base film to improve adherence strength between the polyester base film and an acrylic pressure-sensitive adhesive. This technique, however, requires a very complex treatment process and greatly increases the manufacturing cost. Japanese Patent Publication No. 2006-290993 discloses a method for increasing close adherence to a base film by including methacrylate in a hydroxy group-containing acrylic copolymer. However, this technique fails to exhibit a greater Tg increase than the technique using a carboxyl group-containing monomer. In the above technique, to raise the amount of increase in Tg, an excessive amount of methacrylate monomer has to be added during polymerization. In this case, the molecular weight of the resin cannot be increased much, resulting in degradation of endurance reliability.